Tap the grey box with the plugin icon as shown below to play the Flash content.

Play Flash content without tapping

Keeping your Flash set to Tap to Play is important for your security on the web because it ensures that you decide what plugins are trusted. However, you can customize Firefox for Android to enable Flash by default:

Tap the Menu button (either below the screen on some devices or at the top right corner of the browser)
, then Settings (you may need to tap More first)
, Customize, Display. Then tap the Plugins setting and choose Enabled.

Tap the Menu button (either below the screen on some devices or at the top right corner of the browser)
, then Settings (you may need to tap More first)
, then Display. Then tap the Plugins setting and choose Enabled.